History of animals ~700 million years old



  • Chemical evidence in sediments of fossils that is found in modern sponges


  • Accepted fossils from Ediacaran (pre-Cambrian)
    • mollusks, cnidarians and sponges


  • Animals diversify across Paleozoic, Mesozoic and Cenozoic
    • punctuated by mass extinctions

Animals are monophyletic

  • All share 3 traits


  • All animals, excepts sponges, have:
  1. Nerve cells called neurons
  2. Muscle cells


  • Animals are incredibly diverse
    • 35 major phyla
    • 1/3 are marine
    • most phyla have marine members

Invertebrates: lacking a backbone




  • 95% of known animal species


  • Occupy every habitat on Earth


  • Small to huge


  • Groupings characterized by tissue layers,symmetry and unique structures

Phylum Porifera (Sponges)



  • Most ancestral animal group
    • marine
    • adults do not move
    • filter feeders
    • hermaphroditic (alternate)


  • Lots of cells, but no tissue layers
    • produce many antibiotics


  • Share similarities with protist Chonaflagellates
    • feed using cells called choanocytes

Phylum Cnidarian



  • Animals except sponges are considered “True Animals”
    • contain tissues

Phylum Cnidarian

  • Animals except sponges are considered “True Animals”
    • contain tissues


  • First lineage to diversify were Cnidarians
    • corals, hydras and jellyfish
    • pre-Cambrian


  • Diploblastic and radial symmetry
    • motile and sesile
    • polyp or medusa forms


  • Corals have symbiotic relationships with single-celled algae

Basic Cnidarian body plan ( Nematocyst on phylogeny)

Most animals are triploblastic, bilaterians



  • Triploblasty = 3 germ/tissue layers


  • Complete digestive tract
    • mouth and anus


  • Bilateral = symmetrical in 2 halves

Most animals are triploblastic, bilaterians



  • Triploblasty = 3 germ layers


  • Complete digestive tract
    • mouth and anus


  • Bilateral = symmetrical in 2 halves


  • 3 major clades of bilaterians
  1. Lophotrochozoa
  2. Ecdysozoa
  3. Deuterostomia

Lophotrochozoa: Most share 2 structures




  • Lophophore: crown of ciliated tentacles for feeding



  • Trophophore: distinctive larval stage
    • beating cilia
    • cylinder body


  • This group is falling about with new molecular phylogenies

Lophotrochozoa - 18 total phyla




  • Key phyla (18 total):
  1. Platyhelminthes (flatworms)
  2. Syndermata (rotifers)
  3. Brachiopods (lamp shells)
  4. Mollusca (snails, slugs, oysters, clams, octopuses, squids)
  5. Annelids (segmented worms)


  • Many phyla appeared in Cambrian explosion

Phylum Platyhelminthes (flatworms)

Flattened body = most cells near environment or gastrovascular cavity

Phylum Mollusca: Snails, slugs, clams, mussels, octopus & squid

Phylum Annelida: Earthworms ( segmentation on phylogeny)

Breath through their skin

Ecdysozoans


  • Animals with cuticle
    • shed tough external coat
    • shed as grow; molting; ecdysis


  • 8 total phyla
    • more species than all other groups


  • 2 major phyla
  1. Nematoda (roundworms)
  2. Arthropods (mostly insects)

Phylum Nematoda (round worms)




  • Cylinder bodies from 1mm to 1m


  • Aquatic, soil, plant tissues, animals tissues
    • many parasitic species
    • humans are host to 50 species


  • Longitudinal muscles; thrashing motion

C. elegans: model animal species

Phylum Arthropoda: Rulers of the animal world

  • A billion billion arthropods estimated
    • 1 million species described
    • nearly all habitats on Earth


  • Segmented body, hard exoskeleton, jointed appendages
    • likely led to success


  • Early arthropods (Trilobites) had little variation in body plan


  • Evolution selected for functional body regions
    • specialized functions

Arthropod body plans: unique Hox genes

Phylum Echinoderms: Next time

Invertebrates are not a monophyletic group!